This invention relates generally to mammalian expression systems, and more particularly, relates to mammalian expression systems capable of generating hepatitis C virus (HCV) envelope proteins and the use of these proteins. These HCV envelope proteins, designated as E1 and E2, are fused by removing a cleavage site different from the conventionally observed site. These proteins are expressed in culture medium as well as in mammalian cells.
Hepatitis is one of the most important diseases transmitted from a donor to a recipient by transfusion of blood or blood products, transplantation of organs, and hemodialysis. Viral hepatitis is now known to include a group of viral agents with distinctive viral genes and mode of replication, causing hepatitis with different degrees of severity of hepatic damage through different routes of transmission. Acute viral hepatitis is clinically diagnosed by well-defined patient symptoms including jaundice, hepatic tenderness and an elevated level of liver transaminases such as Aspartate Transaminase (AST) and Alanine Transaminase (ALT).
Non-A Non-B Hepatitis (NANBH) is a term first used in 1975 that described cases of post-transfusion hepatitis not caused by either hepatitis A virus or hepatitis B virus. Feinstone et al., New Engl. J. Med. 292:454-457 (1975). The diagnosis of NANBH was made primarily by means of exclusion on the basis of serological analysis for the presence of hepatitis A and hepatitis B. Currently, NANBH is responsible for about 90% of the cases of post-transfusion hepatitis. Hollinger et al. in N. R. Rose et al., eds., Manual of Clinical Immunology, American Society for Microbiology, Washington, D.C., 558-572 (1986).
The identification of a putative non-A non-B (NANB) agent, Hepatitis C Virus (HCV), has been made. Kuo et al., Science 244:359-361 (1989); Choo et al., Science 244:362-364 (1989). Cloning and sequencing of HCV, now recognized as the primary agent of parenterally transmitted NANBH, has fostered interest and studies in the epidemiology, pathogenesis, and natural history of this disease. Kuo et al., Science 244:362-364 (1989).
Sequences from HCV which encode antigens that react immunologically with antibodies present in a majority of the patients clinically diagnosed with NANBH have been identified. Based on the information available and on the molecular structure of HCV, the genetic makeup of the virus consists of single stranded linear RNA (positive strand) of approximately 9.5 kb, and of one continuous translational open reading frame (ORF) encoding a polyprotein precursor of approximately 3000 amino acids. This precursor protein undergoes cotranslational and posttranslational processing, including cleavage and glycosylation, to the final structural and non-structural proteins. Houghton et al., Hepatology 14: 381-388 (1991). Structural proteins are identified as core protein and highly glycosylated envelope proteins E1 of molecular weight 33,000 and E2 of molecular weight 72,000. Hijitaka et al., Gene 88: 5547-5551 (1991). Replication of HCV occurs early following HCV infection in chimpanzees and a long period of viremia may occur prior to the appearance of antibodies against HCV proteins. Shimizu et al., Proc. Natl. Acad. Sci. USA 87:3392-6444 (1990); Farci et al., New Eng. J. Med. 325: 98-104 (1991).
HCV infection also has been reported in the development of chronic hepatitis, cirrhosis and HCC. Genesca et al., Semin Liver Dis 11: 147-164 (1991). The lack of effective neutralizing humoral immune response to HCV may be related to virus persistence and disease progression. Farci et al., Science 258: 135-140 (1992).
The availability of laboratory tests for serological diagnosis of hepatitis C viral infection has contributed to clarifying the role of HCV in the etiology of hepatitis in patients who have received blood or blood products, or undergone transplantation and hemodialysis. The detection of HCV antibodies in donor samples eliminates 70 to 80% of NANBH infected blood from the blood supply system. However, while the antibodies apparently are readily detectable during the chronic state of the disease, only 60% of the samples from the acute NANBH stage are HCV antibody positive. H. Alter et al., New Eng. J. Med. 321:1994-1500 (1989).
Although assay reagents and methods are available to detect the presence of either HCV antibody and/or HCV RNA, some individuals seropositive for HCV antibody, as well as some individuals infected with the HCV virus, are not diagnosed with HCV by these available assay reagents and methods. For example, it is known that the prevalence of HCV infection is high in kidney transplant recipients; it is hypothesized that active HCV replication may occur in the absence of HCV antibody detectable with current kits. Lau et al., Hepatology 18: 1027-1031 (1993). Moreover, when potential blood donors having a high risk of HCV infection were originally tested with sensitive serological screening assays, 13 of 19 tested were detected by those methods (68%), compared to all 19 blood donors testing positive for HCV RNA by polymerase chain reaction (PCR). Sugitani et al., The Lancet 339: 1018-1019 (1992).
Thus, there is a need for the development of additional assay reagents and assay systems to identify acute infection and viremia which may be present, and not currently detectable by commercially-available screening assays. These reagents and assay systems are needed in order to help distinguish between those individuals with acute and persistent, on-going and/or chronic infection and those individuals whose HCV infections are likely to be resolved, and to define the prognostic course of NANB hepatitis infection in order to develop preventive and/or therapeutic strategies. Also, the expression systems that allow for secretion of these glycosylated antigens would be helpful to purify and manufacture diagnostic and therapeutic reagents.